The properties whereby nematic or nematic-cholesteric liquid-crystalline materials significantly change their optical properties, such as light absorption, light scattering, birefringence, reflectance or color, under the influence of electric fields are utilized to a great extent for electro-optical display elements. The functioning of display elements of this type is based, for example, on the phenomena of dynamic scattering, the deformation of aligned phases, the Schadt-Helfrich effect in the twisted cell or the cholesteric-nematic phase transition.
For the technical application of these effects in electronic components, liquid-crystalline dielectrics are required which must fulfill a large number of requirements. Chemical stability to moisture, air and physical effects, such as heat, radiation in the infrared, visible and ultraviolet ranges, and direct current and alternating electric fields, are of particular importance. Industrially usable liquid-crystalline dielectrics are also required to have a liquid-crystalline mesophase in the temperature range from at least +10.degree. C. to +50.degree. C., preferably 0.degree. C. to 60.degree. C., and as low a viscosity as possible at room temperature, which preferably should not be more than 70.10.sup.-3 Pa.s. Finally, they must not have any characteristic absorption in the visible range, i.e., they must be colorless.
A number of liquid-crystalline compounds has already been disclosed, which fulfill the stability requirements of dielectrics intended for electronic components, and which are also colorless. These include, in particular, the p,p'-disubstituted phenylbenzoates described in German Offenlegungsschrift No. 2,139,628 and the p,p'-disubstituted phenylcyclohexane derivatives described in German Offenlegungsschrift No. 2,636,684. In these two classes of compounds, and also in other known series of compounds with a liquid-crystalline mesophase, there are no individual compounds which form a liquid-crystalline nematic mesophase in the required temperature range from 10.degree. C. to 60.degree. C. As a rule, mixtures of two or more compounds are therefore prepared in order to obtain substances which can be used as liquid-crystalline dielectrics. For this purpose, at least one compound having a low melting point and clear point is usually mixed with another compound having a markedly higher melting point and clear point. This normally gives a mixture whose melting point is below that of the lower-melting component, while its clear point is between the clear points of the components. However, the preparation of optimum dielectrics constantly causes difficulties, since the components having the high melting points and clear points frequently also impart a high viscosity to the mixtures. As a result, the switching times of the electro-optical display elements produced with these mixtures, are extended in an undesirable manner. In addition, problems often occur owing to the fact that the solubility of the various components in one another, particularly at room temperature or lower temperatures, is only very limited.